Journal of Petroleum Science and Engineering xxx (xxxx) xxx Please cite this article as: I.K. Shaik, Journal of Petroleum Science and Engineering, https://doi.org/10.1016/j.petrol.2019.106567 Available online 12 October 2019 0920-4105/© 2019 Elsevier B.V. All rights reserved. Effect of brine type and ionic strength on the wettability alteration of naphthenic-acid-adsorbed calcite surfaces I.K. Shaik a , J. Song b , S.L. Biswal b , G.J. Hirasaki b , P.K. Bikkina a, * , C.P. Aichele a, * a Oklahoma State University, 420 Engineering North, School of Chemical Engineering, Stillwater, OK, 74078, USA b Rice University, 6100 Main St., MS-362, Department of Chemical and Biomolecular Engineering, Houston, TX, 77005, USA A R T I C L E INFO Keywords: Low salinity Carbonate reservoir Enhanced oil recovery Contact angle Calcite surface Naphthenic acid ABSTRACT Low-salinity/smart waterfooding is a technique, used in oil reservoirs, where the salinity and/or ionic composition of the injection water is tuned to improve oil recovery. It has been observed by many researchers that low-salinity waterfooding can enhance oil recovery by altering the wettability of carbonate rock surfaces from oil-wet to water-wet. Though wettability alteration is generally agreed to be the main mechanism for the improved oil recovery, the contributing parameters and necessary conditions for wettability alteration are not clearly understood. Hence, it is essential to decouple the effects of salinity, ionic composition, and oil compo- sition on wettability alteration of the solid surface. In this work, we systematically investigated smart-water induced wettability alteration of naphthenic-acid-adsorbed oil-wet calcite surfaces. Well-characterized model systems were used to understand the effects of individual monovalent and divalent ions (Na þ , Cl  , Mg 2þ and SO 4 2 ), and salinity on the calcite surface wettability. Contact angle measurements were performed on smooth Iceland Spar calcite surfaces that were aged at 120 � C in 5M NaCl brine, acid number 1.5 model oil, and single- electrolyte-based brine solutions of different salinity and ionic composition. The extent of wettability alteration was assessed based on ACA values, which are more relevant during waterfooding, using a goniometer customized for obtaining multiple advancing and receding contact angles along the surface by the tilting plate method. The results indicate that, for 0.164 M ionic strength (equivalent to 4-times diluted seawater concen- tration) brine solutions, the wettability of calcite surfaces was changed from oil-wet state to water-wet state irrespective of the salt type. The lowest advancing contact angle (ACA) values (20 � –30 � range) were obtained for 0.164 M ionic strength NaCl and Na 2 SO 4 solutions, indicating the presence of SO 4 2 ions or reduction in NaCl concentration act in favor of wettability alteration towards water-wet. At this ionic strength, the relative merit of divalent SO 4 2 ions over monovalent Cl  ions was not observed. Upon comparison of ACA values for 0.164 M ionic strength Na 2 SO 4 and MgSO 4 (SO 4 2 ion is common) solutions, we concluded that the extent of wettability alteration is lower in the presence of Mg 2þ ions. A similar conclusion was made by comparing ACA values for 0.164 M ionic strength NaCl and MgCl 2 solutions (Cl  ion is common). The calcite surfaces aged in 0.656 M (equivalent to seawater concentration) NaCl solution demonstrated intermediate-wet state indicating the extent of wettability alteration towards water-wet state increases with a decrease in salinity. 1. Introduction Carbonate rock is naturally water-wet, but, adsorption of polar components such as asphaltenes, resins, and carboxylic acids and/or deposition of the organic material onto the rock surface renders it mixed-wet or oil-wet (Anderson, 1986; Dubey and Waxman, 1991). The extent of wettability reversal of an originally water-wet rock by surface active agents depends on mineral composition, pressure, temperature, pH, brine chemistry and ionic composition (Anderson, 1986). Carbonate rock is positively charged in formation brines with high Ca 2þ and/or Mg 2þ concentration (Jackson et al., 2016), therefore negatively charged acidic compounds such as naphthenic acids in the oil could diffuse through the water flm and chemisorb onto the rock surface (Benner and Bartel, 1941). Several researchers investigated the adsorption of organic acids onto the calcite surface (Gomari et al., 2006a; Gomari et al., 2006b; Hamouda * Corresponding authors. E-mail addresses: imran.shaik@okstate.edu (I.K. Shaik), js110@rice.edu (J. Song), biswal@rice.edu (S.L. Biswal), gjh@rice.edu (G.J. Hirasaki), prem.bikkina@ okstate.edu (P.K. Bikkina), clint.aichele@okstate.edu (C.P. Aichele). Contents lists available at ScienceDirect Journal of Petroleum Science and Engineering journal homepage: http://www.elsevier.com/locate/petrol https://doi.org/10.1016/j.petrol.2019.106567 Received 2 May 2019; Received in revised form 4 September 2019; Accepted 6 October 2019